The furniture manufacturing industry uses as starting material, a large quantity of wood-based panels formed by various industrial methods which enable panels to be obtained satisfying various market requirements. As is well known to the expert in the art, the panels formed in this manner fall substantially into two categories, namely so-called chipboard panels and so-called fiberboard panels, which are also known by the symbol MDF.
Chipboard panels are composed of particles of wood and/or other ligno-cellulose materials, agglomerated by suitable thermosetting resins. These particles are obtained by initial transformation of the raw material into chips of well defined size and thickness which are subdivided to a greater or lesser fineness depending on the compactness of the panel or panel layer to be obtained.
Fiberboard panels are formed from fibers of wood or other lignocellulose materials obtained by the mechanical grinding of the raw material. The procedure is implemented at high temperatures in a pressurized steam environment. As is known to the expert in the art, a medium density fiberboard panel (known in this field as MDF) is formed under dry conditions, with drying of the fibers before forming the so-called "mattress", which is then pressed and treated with thermosetting resins, in the absence of water and under reduced pressure.
Both chipboard panels and fiberboard panels have assumed a fundamental importance in the furniture manufacturing industry because of their workability, the degree of finish obtainable therefrom and their high performance/cost ratio.
The finishing processes (the so-called "enhancement") to which such panels are subjected to give them the characteristics of the final product can be divided into various categories:
a) Enhancement with decorative paper. This process consists of covering the panel with paper which can be colored, or printed with various decorative motifs. This is done by previously impregnating the panel surface with thermosetting resins and then gluing the paper under hot conditions. The paper can also form the base for subsequent coating (discussed hereinafter). A particular type of paper which enables complete uniformity of the panel surface to be achieved is known as "Kraft". This paper is formed starting with the normal paper for such uses, which is then covered with melamine resin serving as a base for a phenolic resin. The paper obtained is usefully used when high surface mechanical characteristics of the product are required. PA1 b) Enhancement with thermoplastic film. A thin film of thermoplastic material, for example, polyvinyl chloride (PVC) is pressed onto the panel, over which glue has been previously spread. By using pressing plates pressed against the surface of the panel covered with the film, special surface effects are achieved which cannot be achieved with other enhancement methods. Moreover, the high foldability of such a film means that the curvatures of the panel ends can easily be followed by the methods known to the expert in the art, and which are briefly described hereinafter. PA1 c) Enhancement by applying precomposed cut sheets (veneering). Thin layers or sheets of wood of various types and various colors are formed by special sophisticated manufacturing techniques. These sheets are glued to the untreated panel to obtain a product having many uses in furniture components. PA1 d) Enhancement by coating. This is a technique which has attained very high quality and is much used in the furniture industry to form panels with a single color surface or covered with transparent coatings. The panel surface first receives an application of a pore sealant (having a predetermined density according to whether the panel is of chipboard or of fiberboard), followed by a levelling coating to smooth the surface, and finally two or more layers of finishing coating to obtain the desired color effect. PA1 1. Nitrocellulose or acrylic-based single-component coatings which dry at ambient temperature by evaporation of the solvent (organic or aqueous); PA1 2. Two-component coatings of acid catalysis alkyd or polyurethane type, which dry at ambient temperature or in hot air (40.degree.-50.degree. C.) by chemical reaction between the functional groups present in the resins; PA1 3. Unsaturated polyester-based coatings containing monostyrene as the reactive diluent (using organic peroxides as catalysts) in the presence of organic salts as activants, which dry at ambient temperature or in hot air (40.degree.-50.degree. C.) by radical polymerization of the unsaturated double bonds present in the resin and in the reactive diluent; and PA1 4. Coatings based on unsaturated polyesters mixed with acrylic unsaturated functionality resins of polyester, polyether, urethane or epoxy type, which when in the presence of particular photosensitive compounds dry by radical polymerization activated by electromagnetic radiation having a wavelength of between 240 and 420 nanometers (ultraviolet UV spectrum). PA1 1. Unsaturated polyester resins dissolved in vinyl or acrylic monomers. These resins consist of mixtures of polycarboxylic acids containing an unsaturated ethylenic double bond (maleic acid, fumaric acid, mesaconic acid, itaconic acid) and/or their corresponding anhydrides, reacted with polyfunctional alcohols (for example ethylene, diethylene, propylene, dipropylene or neopentyl glycol, glycerin, pentaerythritol, trimethylolpropane). The dicarboxylic acids are used in a quantity variable from 10 to 100% (normally from 20 to 80%), and the alcohols in equimolar quantity or slight excess (for example 5%). Difunctional acids of succinic, adipic, azelaic, sebacic, phthalic, orthophthalic, isophthalic or hexahydrophthalic acid type or the corresponding anhydrides can be used in combination with the aforesaid compounds. These acids have the characteristic of not containing reactive ethylenic groups, and are inserted into the structure to modify the physical-chemical properties of the film obtained. The polyesters are generally mixed with vinyl and/or acrylic reactive diluents of the type described hereinafter, in order to obtain a suitable viscosity for use. PA1 2. Epoxy resins with vinyl and/or acrylic functionalization. These resins are condensation products of 2,2-bis-(4,4'-phenol propane) (commonly known as bisphenol A), 1-chloro-2,3-epoxy propane (epichlorohydrin) and acrylic acid. Various components in addition to this structure can be used to modify the physical-chemical characteristics of the resin. Polyfunctional acids such as adipic, succinic or azelaic acid are for example added for this purpose. To achieve the appropriate viscosity for their use, these epoxy resins are mixed with vinyl or acrylic reactive diluents. PA1 3. Polyurethane resins with vinyl and/or acrylic functionalization. These resins are obtained by reacting other hydroxylated molecules with diisocyanates and particular molecules having an unsaturated functionality and a hydroxyl functionality. The polyols used can comprise the following compounds: polyethylene and polypropylene glycols of different molecular weight, diols of neopentyl glycol or hydroxypivalic type, triols such as trimethylolethane or propane, or glycerol; hydroxylated low molecular weight polyester resins, polyesteramides obtained by adding cyclic ketones to diols. The most frequently used isocyanates contain two --NCO groups per molecule and include 2,4-2,6-toluenediisocyanate, 1,6-4,4'-diphenylmethanediisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 1,6-hexamethylenediisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 2,2,4-trimethylhexane -1,6-diisocyanate. The reactive functionality of ethylenic unsaturated type is introduced by hydroxylated unsaturated molecules, the unsaturation being of hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate or corresponding methacrylic derivative type. In the case of vinyl unsaturation, hydroxylated molecules such as 4-hydroxybutyl vinylether are used. These resins are particularly important in that by virtue of the particular structure of the bonds present (urethanic) they give the coating film superior elasticity and hardness. PA1 a) Acrylic esters of different viscosity and functionality. These molecules perform a double function when included in the composition of a product curable by ionizing radiation. In this respect they serve both to give the basic resin those characteristics enabling a coating film to be obtained having the desired final properties, and as reactive diluents used to adjust the product to a suitable viscosity for application. In general, they can be acrylic or methacrylic esters or amides, or comonomers of these esters with other copolymerizable monomers. For example, linear chain alcohol esters of methacrylate, methylmethacrylate, ethylacrylate, butylacrylate or 2-ethylhexylacrylate type can be used. The possible amides include acrylamide, tert-butylacrylamide and primary alkylacrylamides. Molecules of other type can be used to obtain diluents of unsaturated functionality having the required characteristics, and in particular, itaconic esters; maleic esters; compounds containing alkyl groups; diol or triol acrylates and methacrylates such as 1,6-hexanediol, neopentyl glycol, 1,4-butanediol, trimethylolpropane, pentaerythritol, acrylates of oxyethylene and oxypropylene derivatives of various degrees of compensation and molecular weight, low molecular weight polyester acrylates obtained by condensing dicarboxylic acids and polyols (for example, adipic acid, azelaic acid, phthalic acids and corresponding anhydrides with ethylene or propylene glycols of various molecular weights, or saturated alkylene diols such as 1,6-hexanediol, trimethylolpropane). PA1 b) Compounds containing vinyl groups. These are used mainly as "reactive diluents", their purpose being to adjust the coating to the desired application viscosity. Examples of such compounds are: vinylacetate, styrene, vinyltoluene, divinylbenzene, methylvinylether, ethylvinylether, butylvinylether, tripropyleneglycol divinylether, diethyleneglycol divinylether, 1,4-butanediol divinylether, tetraethyleneglycol divinylether.
The type of enhancement described under points a) and c) above can be completed with a transparent finish obtained by applying a transparent base coating to smooth the surface, followed by one or more layers of finishing coating to give the panel the desired gloss effect. These coatings are applied in various ways, depending on the form of surface to be coated. The method of application mostly used is spraying by manual spray guns or by using robotized equipment which also enables the curved ends of the panel to be coated. Roller spreading or curtain coating machines are known which, although allowing mass production, only enable the flat surface of a series of panels to be coated (not their ends). The coating thicknesses to be applied can vary within a very wide range, depending on the type of panel to be coated and the coating product used.
The coating products conventionally used for coating the aforedescribed panels can be divided into the following categories:
The use of coatings of points 1 and 2 above has the drawback that they contain organic solvents and hence contrast with the modern industrial tendency of not using pollutant products, but instead using those with an extremely low or zero content of volatile organic substances (VOS).
Although the coatings of point 3 above have a significantly reduced VOS content, they contain a harmful reactive diluent of low vapor pressure (styrene). They are also unsuitable for automated coating cycles because of their relatively low polymerization rate.
The coatings of point 4 are currently the industrially most advanced of the traditional coatings, in that they can be used in automated production, although at a production rate which is not yet high. This is due to the fact that the dangerous reactive diluent can be limited by replacing it with other diluents of the acrylic type (i.e., containing an unsaturation derived from acrylic acid) which have a much higher vapor pressure.
As is known to the expert of the art, photoactive curing allows rapid drying of these coatings is of a transparent type, i.e., formed from components which do not act as a filter against electromagnetic radiation. However, if organic and/or inorganic pigments are introduced in order to obtain colored coatings, curing is strongly retarded. Hence the quantity of pigment has had to be limited to a low percentage (not exceeding about 10% by weight), with the result that these coatings have a limited covering power. To obviate this drawback, multiple layers of such coatings have recently been used so as to divide the pigment between them and reduce their screening effect, or alternatively coatings of the types 1, 2 or 3 have been combined with those of type 4. In the first case, a multi-layer pigmented covering is achieved having a large total thickness and low reactivity towards UV. This means that in practice resins with a high density of reactive groups have to be used, with the result that the multi-layer coating is fairly rigid both intrinsically and because of its large thickness. Up to the present time, this has precluded the use of pigmented coatings of type 4 for enhancing panels with rounded edges by the so-called preforming or direct postforming process which require the already cured coating layer to be bent to cause it to adhere to the curved end.
In the case of the combination of a type 1, 2 or 3 coating with a type 4 coating, the already described ecological and economical drawbacks apply.
For a better understanding of the ensuing description, it is considered appropriate to briefly describe the methods for completing panel enhancement on their ends, these being substantially of two types:
I. Manual methods.
These methods are used at the craftsman level and consist of manually covering the panel end with strips of the most varied materials such as wood, plastic and in particular PVC (polyvinylchloride) or ABS (acrylonitrile-butadiene-styrene copolymer), hide, glass or metal. This enables panels to be obtained having ends which satisfy the most varied decorative requirements.
II. Industrial methods.
These methods are implemented completely automatically. Their greatest limit is that for end enhancement they enable only a limited number of materials to be used, the thickness of which is limited to a narrow range, as is also the end height. In particular, for PVC and ABS ends the thickness must be between 0.2 and 0.3 mm, for melamine laminate ends the thickness must be between 0.2 and 0.8 mm, and for wood strips the thickness must be between 0.2 and 25.0 mm. These methods consist of "adding", i.e., gluing, along the panel ends a strip of one of the aforelisted materials, possibly after previously rounding the panel end by soft-forming. A further finish can be applied to the end obtained in this manner, for example a coating, if a wooden strip has been used.
The requirement for qualitatively and aesthetically improving the finished edged panel has led to the conception of an industrial process known as postforming, which achieves the important result of obtaining a panel of uniform appearance (i.e., the end has the same appearance as the rest of the panel), thereby avoiding anti-aesthetic discontinuities which can also represent paths for the penetration of moisture from the outside, compromising the final product even a short time after its manufacture.
For a better understanding of the present invention, a brief description will be given of the stages involved in postforming. Reference will be made to FIGS. 1-5 of the accompanying drawings, which show a partial cross-section through a rounded-end panel during the various stages of implementation of this enhancement procedure. Specifically, FIG. 1 shows the right end portion of an untreated chipboard or fiberboard panel 10 to be enhanced, the relative end 12 having a semicircular profile. FIG. 2 shows the same panel 10, to the two faces of which there has been applied a respective sheet (14 and 15) consisting of paper or a film of thermoplastic material (such as PVC). As can be seen, the sheet 14 applied to the upper face projects beyond the end of the panel 10 for a precise predetermined distance (its projecting edge possibly being suitably ground by an appropriate tool 16, as shown in FIG. 3), whereas a sheet 18 applied to the lower face of the panel 10 has only a minimum projection. As shown in FIG. 4, a tool (represented very schematically in this figure and indicated by the reference numeral 20) is used to remove from the lower sheet 18 an end strip of suitable length such that when the projecting part of the upper sheet 14 is bent against and into contact with the rounded end 12 of the panel 10 while being simultaneously hot-glued, this entire end 12 becomes covered by the upper sheet 14 (as can be seen in FIG. 5). Only a joining line 22 which separates the upper sheet 14 bent around end 12 from the lower sheet 18 remains visible in the finished panel 10'.
An improvement on the aforedescribed postforming process is the so-called preforming process, also known as direct postforming, which compared with the preceding has considerable production and cost advantages. In this respect, it starts with a standard finished panel, i.e., already enhanced but only on its faces. This hence dispenses with one specific panel enhancement stage, as instead is required in the preceding case.
A brief description will be given of a known preforming process with reference to FIGS. 6-14 of the accompanying drawings, which show a partial cross-section through a standard panel. This panel is shown during the successive stages of implementation of the process, its right end being flat, vertical and not enhanced. Specifically, FIG. 6 shows a panel 30, already enhanced by the application on each of its two faces of a sheet, 34 and 38, respectively, of decorative paper (Kraft or melamine type) or a film of thermoplastic material. The same figure also shows the formation in the upper surface of the panel, at a suitable precalculated distance from an end 32 and by means of a suitable cutting tool 36, of an incision extending perpendicular to the plane of the sheet and having a depth greater than the thickness of the upper sheet 34. Using a milling tool 40, both that portion of the upper sheet between the incision and the panel end 32 and a large part of the underlying panel portion are then removed (FIG. 7). Using another suitable tool 46, the remaining lower portion of the panel is then also removed, practically as far as the lower sheet 38 (FIG. 8) whereby the lower sheet 38 consequently projects a certain distance from the panel 30. The panel now has a new side 32' (which need not be vertical, but can be inclined to the panel faces) to the rear of the original side 32. Using a further suitable tool 48, the lower edge of the side 32' is then rounded, to obtain a partially curved side 32" (FIG. 9). This rounding can have a maximum radius of curvature equal to one half the panel thickness (it is smaller in the case illustrated). Using a further tool 49, an incision is made between the lower end of the rounding and the lower sheet 38, to obtain a side 32"' shaped as in FIG. 10. If the projecting lower sheet portion is now bent upwards and glued against the side 32"', the panel 30 of FIG. 11 is obtained, in which the only discontinuity is the joining line 42 between the sheet 34 and the sheet 38.
If a panel is required with an end having both edges rounded, the stage shown in FIG. 10 is followed by the further stages shown in FIGS. 12-14, comprising removing a further end portion of the sheet 34 using a suitable tool 50 (FIG. 12), then rounding the upper edge of the panel 30 using a further tool 52 to obtain the end 32"" with double rounding, then grinding the edge of the lower sheet 38 using a suitable tool 54 (FIG. 13), and finally bending the projecting portion of this lower sheet 38 upwards and hot-gluing it against the end 32"" to obtain the finished panel 30" of FIG. 14.
Obviously, if a rounded end such as that shown in FIGS. 1-5 is required, the radius of curvature with which the two edges of the panel are rounded is equal to one half the thickness of the panel 30.
A machine normally used for implementing the aforedescribed process in a completely automatic manner is that manufactured by the German firm Homag Maschinenbau AG, carrying the symbol VFL.
In both the described postforming process (FIGS. 1-5) and preforming or direct postforming process (FIGS. 6-14), the covering sheet which is bent and glued to the curved end is subjected to high-intensity thermomechanical stressing (a temperature of up to 200.degree.-250.degree. C. is used), because of which the sheet used must have particular structural characteristics to obtain a homogeneous result without splitting or color changes. Panels covered with melamine sheet have proved suitable for the purpose, whereas coated panels using traditional coatings and processes have demonstrated problems in resisting the mechanical stresses to which they are subjected during the process, with consequent microscopic or even macroscopic fractures arising. For these reasons, up to the present time it has not be possible to produce rounded-end panels coated by an automated industrial process, such panels being necessarily produced by the aforedescribed manual craftsman method.